Fuel management system

ABSTRACT

An aircraft fuel management system providing accurate and comparable measurement and display of on-board fuel quantity, fuel rate and flight time remaining. This system utilizes capacitance method of fuel gaging and fuel mass flow rate derived from both volumetric flow measurement and capacitance density compensation. These separate measurements - fuel gaging and mass flow rate - are combined electronically to obtain &#39;&#39;&#39;&#39;flight time remaining&#39;&#39;&#39;&#39; under prevailing flight conditions.

O United States Patent 1 1 1111 3,739,635 Stuart 1 June 19, 1973 FUELMANAGEMENT SYSTEM Prima Examiner-Richard C. Queisser 751 t.D lE.SttMddlb ,Vt. 1 men or oug as I e my Assistant Examiner-Stephen A.Kreitman [73] Assignee: Simmonds Precision Products, Inc., Au .Ed i E, Gi

Tarrytown, N.(.

[22] Filed: May 12, 1971 [57] ABSTRACT [21] Appl. No; 142,525

An aircraft fuel management system providing accurate and comparablemeasurement and display of ongf 5 g g board fuel quantity, fuel rate andflight time remaining. 73 194 195 This system utilizes capacitancemethod of fuel gaging 1 0 l and fuel mass flow rate derived from bothvolumetric flow measurement and capacitance density compensa- [56]References cued tion. These separate measurements fuel gaging and UNITEDSTATES PATENTS mass flow rate are combined electronically to obtain2,981,105 4/1961 Ryder 7.3/304 flight time remaining under prevailingflight condi- 3,050,999 8/1962 Edwards 73/304 ti s FOREIGN PATENTS ORAPPLICATIONS 2 Cl 4 D F 817,009 7/1959 Great Britain 73 194 M rawmg Z?f/fi/ifrflzgififl I115: flaw/r) 1 mi 73 077 54 4/:

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I r A 4.? flaw/ AW! ,4 main 742w! RIGHT ENGINE FUEL VOLUME FLOW RATEMONITOR III Illlll MODULATOR-S [MODULATOR] MODULATOR] FUEL DENSITYMONITOR VOLUME FLOW RATE MONITOR LEFT ENGINE 2511. MODULATOR 4OOHZ REEINVENTOR DOUG LAS E. STUART ATTORNEY SIEI 1 U 2 DENSITY MONITOR IF +F IIMODULATOR IQ I SERVO AMPLIFIER VOLUME MONITOR COMPARATOR CHOPPER RI FRZ0 m n m JIIII 1 sun FIG. 2

IBIQR) FIG. 3

FUEL MANAGEMENT SYSTEM BACKGROUND OF THE INVENTION DESCRIPTION OF THEPREFERRED EMBODIMENT Accurate and com arable measurement d di l 5 andfuel flow gage 12 are connected to the flight time of the fuel quantityon board, fuel consumption rate, and flight time remaining hasrepresented a serious problem in the past. Fuel quantity and flow ratemeasurement were expressed in different units or the measurementdiffered because of variations in conditions at the points ofmeasurements. These handicaps required the use of elaborate conversionand/or correction techniques to provide comparable information.

Too, previous attempts at presenting actual flight time remaining onboard aircraft were dependent upon the accuracy of two differentsub-systems usually made by separate manufacturers with errors resultingfrom one of the sub-systems measuring volume and not mass, and resultingfrom a sub-system not compensating for density changes or simply havingtemperature compensation. These errors very often were quite high.

The present system provides the information accurately and in comparableunitsby utilizing the capacitance method of fuel gaging and the fuelmass flow rate derived from both volumetric flow measurement andcapacitance density compensation. These separate measurements arecombined electronically to obtain flight time remaining under prevailingflight conditions.

Accordingly, an object of this invention is to provide accurate flighttime remaining under prevailing flight conditions.

Another object of this invention is to provide a system of fuelmanagement which provides the needed information accurately and incomparable units utilizing straight forward measurement principles.

Another object of this invention is to provide a capacitance method offuel gaging and fuel mass flow rate derived from volumetric flowmeasurement andcapacitance density combinations for accurate display. ofaircraft flight time remaining.

' BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of theapplication of the invention showing the combination of the fuelquantity signals and rate signals to display flight time remaining;

FIG. 2 is a schematic diagram used herein to illustrate the devicesoperationally tied into the fuel time remaining gage; and

FIG. 3 is a schematic illustration of thercapacitance compensator flowmeter in line with the fuel flow;

FIG. 4 is a schematic illustration of the overall system showing how thefuel line to a given engine is monitored as to its volume flow rate andas to its fuel density to provide a mass flow rate signal which issummed with remaining gage 14. Fuel quantity signals 16 and 18,displayed on the fuel quantity gage 10, and fuel flow signals 20 and 22displayed on the fuel flow gage 12 are combined on the flight timeremaining gage 14.

Quantity and density monitors 24 and 26, per se, do not form a part ofthis invention and therefore it will suffice only to summarily describetheir operation. A detailed description can be had by reference to U. S.Pat. No. 2,981,105 issued to F. L. Ryder on Apr. 25, 1961 and U. S. Pat.No. 3,050,999 issued to H. F. Edwards on Aug. 28, 1962.

As described in the foregoing patents, the liquid quantity monitor 24comprises a number of capacitance sensors or measuring probes immersedin a fuel tank 28. The probes are connected to a fixed, highly stablereference capacitor in a bridge arrangement with bridge nulling by afeedback potentiometer so that any change in the capacity of theimmersed measuring probe due to the change in the fuel in the tank willbe sensed and rebalanced continuously.

Tank 28 is shared with a density monitor 26, which also is a bridgecircuit similar to the quantity monitor 24, to monitor changes in thefuel dielectric constant which is proportional to density.

With suitable transformers, high and low pass filters, the two monitors24 and 26 may be frequency multiplexed.

The mass flow rate system shown in FIG. 1 is for two engines 30 and 30'connected to the fuel flow gage 12. The system comprises a monitor 32and 32' and a turbine fuel flow meter 37 connected upstream of theengine 30.

The monitor 32 derives a signal. representative of a fuel density and isobtained through a capacitance type capacitor which consists of tubecapacitors 36 located in the fuel line 38 as shown in FIG. 3. Themonitor 32 is similar to the monitor described in the fuel gage systems24 and 26 in tank 28 and continually senses the capacitance value of thefuel in the fuel line so that variations in the fuel dielectricproperties are always detected. The tube capacitors 36 like thecapacitors of the fuel gage monitors '24 and 26, but located in the fuelline, are connected to a stable reference capacitor in a bridgearrangement with a bridge nulling being a feedback potentiometer so thatany change in the capacity of the tube capacitors due to a change in thedensity of the fuel in the line 38 is sensed and rebalanced. Again,since this monitor 32 does not form a part of this invention, it willsuffice only to summarily describe its operation; a detailed descriptionof similar monitors of the type used in fuel gaging can be had byreference to the aforementioned patents; the differences being only thatthe capacitors are in the fuel line rather than in the tank.

The volume flow rate signal is obtained through a reluctance sensorprobe 40 which senses the rotation speed of a turbine wheel 42 as shownschematically in FIG. 3 and responds quickly to flow variations and solocated that the fuel flow causes the wheel to turn at a speedproportional to flow velocity. This rotation is sensed by the sensorlocated adjacent the turbine but outside the fuel environment.

In the embodiment shown, the volume fiow rate signal obtained throughthe reluctance sensor 40 which senses rotation of the turbine wheel 42modulates a 45 khz carrier signal applied to the sensor. The modulatedcarrier signal representing the volume flow rate is then processedthrough a signal modulator where it is demodulated and transferred intoa 25 volt square wave whose frequency is representative of the volumeflow rate. Use of the demodulated carrier principle eliminates magneticdrag and provides high amplitude pulses of a low flow condition which,in turn, assures high accurate measurement in the low flow range.

The signal representative of the fuel density obtained through thecapacitance monitor 32 as aforesaid is similar to those currently usedin the quantity gaging systems 24 and 26. This signal may have a 16 khzapplied to the compensator which is processed through an amplifier intoa demodulator to an 8 volt rms signal representing density variations.

As aforesaid, since it is very important to measure the flow of the fuelto the engines and since the density of the fuel both at the tank and atthe engines is important otherwise an error as much as percent willresult, the two signals generated from the monitors in the tank and inthe fuel lines may be combined to give a flight time remaining.

In FIG. 2 the operation of the flight time remaining gage 14 is based onthe equation:

Time remaining t (Q Q )/(F F (Q Q )(F F (t 0 error signal, where Qrepresents the quantity of fuel in the left tank, Q represents thequantity of fuel in the right tank; F represents the flow from the lefttank; F R represents the flow from the right tank; and t represents thetime remaining. The sum of the two flow rate signals and 22 (from theleft engine and the right engine) from the density and volume monitorsare summed through amplifier 44 and appear as a reference voltage on therebalance potentiometer 46, the displacement potentiometer shaft 48 isgeared directly to the time remaining counter 14. The rebalance signalis then (F F (1 when compared to the signals Q, Q, combined throughsumming amplifier 50 to null the servo system thus solving the equationQ Q As will be seen in FIG. 4, for example, the left engine has a fuelline 38 which carries fuel to it. The fuel flowing in line 38 ismonitored by the volume flow rate monitor 37 and the fuel densitymonitor 32. These elements have been previously described in connectionwith FIG. 3 wherein the turbine fuel flow monitor 37 is shown having aturbine wheel 42, a reluctance sensor probe 40 and a modulator whichconverts the volume flow rate into a square wave whose frequency isproportional to the volume flow rate. The fuel density monitor orcapacitance monitor 32 was also shown in FIG. 3 wherein the fuel densityis converted by a modulator to a voltage which is representative -of thefuel density.

Again referring to FIG. 4, the fuel density monitor signal and the fuelvolume signals are combined in another modulator to provide an outputsignal 20 which represents the'total mass flow rate through the fuelline to the left engine 30. The signal 20 is fed to amplifier 44 whereit is summed with a mass flow rate signal 22, for example, from theright engine. The mass flow rate from any other engine could also beinserted as a signal into summing amplifier 44 also. The summed outputsof amplifier 44 provide a voltage or potential which is placed acrossthe potentiometer 46 and the voltage across the potentiometer 46represents the total mass flow rate being fed to all of the engines ofthe airplane or other vehicle.

Concomitantly there is another system in operation. There is seen a fueltank 28 which in this case represents the left fuel tank. Within thisfuel tank is a fuel volume monitor 24 and a fuel density monitor 26which operate according to the prior discussion aforementioned underFIG. 1. The signal outputs of 24 and 26 are combined to give a signalrepresentative of the fuel quantity in the left tank, and designated at16 (Q This signal 16 is fed into an amplifier 50 which may also receivesignals representative of fuel quantity in the right tank, which signalline may be designated as 18 (Q Thus the signal output of the amplifier50 represents the sum total of fuel quantity in all the tanks, at anygiven time.

The signal from amplifier 50 is fed into one side of a comparatorchopper which has another input taken from the movable slider portion ofthe potentiometer 46, in a system which operates to cause a balancingaction to take place so that the potential taken from the slider ofpotentiometer 46 becomes automatically matched to the signal output fromamplifier 50. As this balancing action takes place through motor 18which drives the link 48 to cause the slider of potentiometer 46 to pickoff a signal potential equal to the signal potential of amplifier 50,the corrective motor action at the same time is used to turn anindicator 14 which indicates the quantity known as the flight timeremaining. Thus by measuring the sum total of the mass flow rate of fuelbeing consumed by one or all of the engines (as converted to anelectronic signal across the potentiometer 46) as against the signalrepresentative of fuel quantity in one or all of the fuel tanks, theequations, previously indicated in regard to eliciting the timeremaining, are satisfied.

From the foregoing, it can be seen that there is disclosed a fuelmanagement system improvement to todays aircraft operation. One simpledisplay (shown digitally in FIG. 2) tells the pilot the length of timehe can continue to fly at the prevailing throttle setting. This systemis accurate, is economical because of the combination of the fuel flowrate and the fuel gaging systems combined relatively simplyelectronically, is safe since the accuracy of the entire system isenhanced and is reliable because of its simplicity of design.

That which is claimed is:

1. A system for measuring the flight time remaining for an aircrafthaving a fuel tank and at least one engine comprising:

monitoring means located in the fuel tank and capacitatively responsiveto the dielectric of the fuel so as to generate a signal responsive tothe volume of the fuel;

monitoring means located in the fuel tank and capacitatively responsiveto the dielectric of the fuel in the tank to generate a signalresponsive to the density of the fuel;

monitoring means located in the fuel line upstream of the engine andcapacitatively responsive to the dielectric of the fuel so that a signalis generated responsive to the density of the fuel; and

monitoring means in the fuel line upstream of the engine and responsiveto the flow of fuel therethrough so as to generate a signal indicativeof the volume of fuel flowing through the flow line and cator togenerate the flight time remaining indication.

2. The system claimed in claim 1 further including means coupling saidfuel tank monitoring means to coupled to said density monitor so as togenerate 5 generate a signal indicative of the fuel mass in the tank,

a signal indicative of the mass flow rate of fuel through said flowline; and

abridging network combining the output signals of all of the monitoringmeans and coupled to an indirate of fuel mass consumption by the engine.

1. A system for measuring the flight time remaining for an aircrafthaving a fuel tank and at least one engine comprising: monitoring meanslocated in the fuel tank and capacitatively responsive to the dielectricof the fuel so as to generate a signal responsive to the volume of thefuel; monitoring means located in the fuel tank and capacitativelyresponsive to the dielectric of the fuel in the tank to generate asignal responsive to the density of the fuel; monitoring means locatedin the fuel line upstream of the engine and capacitatively responsive tothe dielectric of the fuel so that a signal is generated responsive tothe density of the fuel; and monitoring means in the fuel line upstreamof the engine and responsive to the flow of fuel therethrough so as togenerate a signal indicative of the volume of fuel flowing through theflow line and coupled to said density monitor so as to generate a signalindicative of the mass flow rate of fuel through said flow line; and abridging network combining the output signals of all of the monitoringmeans and coupled to an indicator to generate the flight time remainingindication.
 2. The system claimed in claim 1 further including meanscoupling said fuel tank monitoring means to generate a signal indicativeof the fuel mass in the tank, and further including means coupling saidfuel line monitoring means to generate a signal indicative of the rateof fuel mass consumption by the engine.